The present invention relates generally to fiber optic directional couplers, and more specifically, to devices which selectively couple light energy between the modes of an optical fiber.
Fiber optic directional couplers provide for the transfer of optical energy traveling in one fiber optic waveguide to another. Such couplers are useful for a variety of applications, e.g. in fiber optic sensors.
As is well known in the art, a single optical fiber may provide two or more waveguides under certain conditions. These waveguides are commonly referred to as the normal modes of a fiber, which may be conceptualized as independent optical paths through the fiber. Normal modes have unique electric field distribution patterns which remain unchanged, except for amplitude as the light propagates through the fiber. Additionally, each normal mode will propagate through the fiber at a unique propagation velocity.
The number of modes which may be supported by a particular optical fiber is determined by the wavelength of the light propagating therethrough. If the wavelength is greater than the "cutoff" wavelength, the fiber will support only a single mode. If the wavelength is less than cutoff, the fiber will begin to support higher order modes. For wavelengths less than, but near cutoff, the fiber will support only the fundamental, or first order mode, and the next, or second order mode. As the wavelength is decreased, the fiber will support additional modes, e.g. third order, fourth order, etc. These first order, second order, third order, etc. normal modes are commonly referred to as the "spatial" modes of the fiber.
Each of spatial the normal modes (e.g. first order, second order, etc.) are orthogonal, that is, there is no coupling between the light in these modes. In addition, each of the spatial normal modes includes two orthogonal polarization modes, which may be defined e.g. as the linear vertical polarization mode and the linear horizontal polarization mode. The orientation of the electric field vectors of the modes defines the polarization of the light in the mode, e.g. linear vertical, or linear horizontal. A more complete discussion of these modes, and their corresponding electric field patterns, will be provided below.